CHICAGO -- The Minnesota Vikings were just lucky enough to escape Soldier Field with a 20-17 win over the Bears on Monday Night Football. But once again, they weren't as fortunate when it came to injuries.

Sam Bradford returned after missing the Vikings' past three games with a noncontact knee injury sustained in the season opener and couldn't finish the first half. He was sacked four times throughout and at some point re-aggravated the injury in his left knee, forcing him to exit the game.

"I just thought it was best for him that we got him out of there," Vikings coach Mike Zimmer said. "He didn’t want to come out. He wanted to stay in there and fight, but I didn’t want to get him injured anymore."

Zimmer said he thought about pulling Bradford earlier but the quarterback felt "pretty good about things."

Bradford had only participated in a limited capacity heading into the Vikings' second straight NFC North matchup after missing two weeks of practice. Still, Zimmer had said there was "no doubt" Bradford could play in Monday's contest and even cited the Vikings practicing on turf two days last week as a reason they had confidence in his ability to return.

"We wouldn’t have put him on the field if he wasn’t healthy enough to play," he said. "He was very confident about it, felt good about it. Medical people felt good about it. Everybody."

Bradford told reporters last week that he felt good after receiving a second opinion on his injured knee on Sept. 22. The good news he received from doctors was that whatever was causing his knee issues (he would not specify) "should be something that eventually goes away."

Yet Monday’s first half was indicative that Bradford was not ready to return. From the very outset, Bradford did not look right, and he was benched with 25 seconds remaining in the second quarter.

On the Vikings' first two drives, the quarterback overthrew Stefon Diggs twice and Adam Thielen once. Wearing a brace on his left knee underneath his uniform, Bradford was hesitant while running and on his throws. He was sacked four times for a loss of 35 yards, but it was hardly because the Bears wore down his offensive line. Chicago recorded all four of its sacks when sending four or fewer pass-rushers, per ESPN Stats & Information data.

Bradford said he expected to have to play through some discomfort, but it was evident that the pain in his left knee was holding him back.

On the first sack, which resulted in a safety for Chicago, the Vikings were backed up at their own 5-yard line after being slapped with a holding penalty on a punt return. Bradford gingerly moved around in the pocket upon dropping back and waited too long to make a throw before Leonard Floyd came around the quarterback’s right side and pulled him down in the end zone for two points.

The second sack was by Akiem Hicks and occurred when Hicks made contact around Bradford's waist but did not tackle him to the ground. Instead, Bradford appeared to spin and lose his footing before falling forward.

The third was for a 15-yard loss.

And on the fourth sack, Bradford appeared to twist his knee. On that play, he backed into center Pat Elflein and fell backward. He walked off the field noticeably limping and was pulled for the Vikings' final drive of the half.

Bradford finished the night 5 of 11 passing for 36 yards.

In his absence, Case Keenum bailed out the Vikings and proved why he should have started. Minnesota's veteran backup was 17-of-21 for 140 yards and a 13-yard touchdown to Kyle Rudolph.

Jerick McKinnon provided the boost needed in the run game, surpassing Latavius Murray in carries. McKinnon carried 16 times for 95 yards and a touchdown.

The drama surrounding the Vikings' quarterback situation will grow in a matter of days. Teddy Bridgewater is eligible to come off the physically unable to perform list following the Week 6 game against the Green Bay Packers. Reports have indicated that Bridgewater has not experienced setbacks in his recovery and should be ready to practice once that window opens.

But practice is all the action Bridgewater will see, at least early. Minnesota has three weeks to determine whether he'll be activated to the 53-man roster this season.

With the possibility of two starting quarterbacks competing in practice, neither of whom is currently signed past this season, there is intrigue regarding what the Vikings will do in the immediate future and beyond.

Zimmer cited pain management as the root of Bradford's current knee problems. Both the Vikings and Bradford have said this issue is not expected to be long term.

"I’m still hopeful with him," Zimmer said. "Everything that’s been from the medical people, the doctors, the second opinion, everything is it’s going to get better. I just think he aggravated it a bit on one of those. We’ll take every day, day by day, and see how it goes. I think he’s going to get back and he’s going to get better."

But after his Monday night performance, and with what could happen the next several weeks, Bradford’s job could be in jeopardy when Bridgewater returns.